We report the discovery of a very cool , isolated brown dwarf , UGPS 0722-05 , with the UKIDSS Galactic Plane Survey .
The near-infrared spectrum displays deeper H _ { 2 } O and CH _ { 4 } troughs than the coolest known T dwarfs and an unidentified absorption feature at 1.275Â \mu m. We provisionally classify the object as a T10 dwarf but note that it may in future come to be regarded as the first example of a new spectral type .
The distance is measured by trigonometric parallax as d =4.1 _ { -0.5 } ^ { +0.6 } Â pc , making it the closest known isolated brown dwarf .
With the aid of Spitzer /IRAC we measure H - [ 4.5 ] = 4.71 .
It is the coolest brown dwarf presently known – the only known T dwarf that is redder in H - [ 4.5 ] is the peculiar T7.5 dwarf SDSS J1416+13B , which is thought to be warmer and more luminous than UGPS 0722-05 .
Our measurement of the luminosity , aided by Gemini/T-ReCS N band photometry , is L=9.2 \pm 3.1 \times 10 ^ { -7 } L _ { \odot } .
Using a comparison with well studied T8.5 and T9 dwarfs we deduce T _ { eff } =520 \pm 40Â K. This is supported by predictions of the Saumon & Marley models .
With apparent magnitude J = 16.52 , UGPS 0722-05 is the brightest brown dwarf discovered by UKIDSS so far .
It offers opportunities for future study via high resolution near-infrared spectroscopy and spectroscopy in the thermal infrared .